1. Field of the Invention
The present invention relates to protection of a power conversion device.
2. Description of the Related Art
It is known that a power conversion device may be constituted by a bridge connection of a circuit consisting of semiconductor power elements (for example thyristor, gate turn-off thyristor and so on) and feed back diodes connected in antiparallel with these semiconductor power elements. Fuses are connected in series to those power semiconductor elements in order to protect the power semiconductor elements from overcurrent.
FIG. 1 is a layout diagram of a conventional power conversion device 1. A circuit consisting of power semiconductor elements 2a, 2b, 2c, and 2d, and fuses 4a, 4b, 4c, and 4d respectively connected in series therewith
and respective feedback diodes 3a, 3b, 3c, and 3d connected in antiparallel with these semiconductor power elements 2a, 2b, 2c and 3d is connected in a bridge arrangement. The A.C. terminals 5a, 5b of this bridge circuit are connected to an A.C. power source 9 through an A.C. circuit breaker 6. The D.C. terminals 8a and 8b are connected to a D.C. power source provided with a capacitor 10. The D.C. power source is shown only by a capacitor, but it could be connected for example to a D.C. circuit such as a rectification device that converts A.C. to D.C.
In the power conversion device 1 as shown in FIG. 1, D.C. power can be converted into A.C. power by repetition of the following operation: semiconductor power elements 2a and 2d are turned ON, then after turning semiconductor power elements 2a and 2d OFF, semiconductor power elements 2b and 2c are turned ON, and then after turning semiconductor power elements 2b and 2c OFF, semiconductor power elements 2a and 2d are turned ON. If, during the operation described above, for example semiconductor power elements 2b and 2d are simultaneously turned ON due to a malfunction of the semiconductor power elements or to a failure of commutation etc., the discharge current from capacitor 10 becomes a short-circuit current flowing as shown by the broken line in FIG. 1. Fuses 4a-4d are provided to protect semiconductor power elements 2a-2d from such a short-circuit current.
As described above, apart from malfunctions in which the semiconductor power elements constituting the upper side are simultaneously turned ON, D.C. short-circuiting can occur, such as short-circuiting of capacitor 10, even when the semiconductor power elements are functioning normally. In such a case, if the A.C. side terminal 5a is positive and the A.C. side terminal 5b is negative, the short-circuit current flows by the path indicated by the continuous line in FIG. 1.
In contrast to this, if the A.C. side terminal 5a is negative and the A.C. side terminal 5b is positive, the short-circuit current flows through fuses 4b and 4c.
Fuses 4a-4d are provided with the object of protecting semiconductor power element 2a-2d from overcurrent, so they have a high-speed melting characteristic, so that the fuses melt before A.C. circuit breaker 6 opens (a time of about three cycles is required for A.C. circuit breaker 6 to open). Consequently, when D.C. short-circuiting other than a failure of commutation occurs, even though the short-circuit current doesn't flow through the power semiconductor elements, the power conversion device must be temporarily stopped in order for the fuses to be changed, because the fuses are melted by the short-circuit current.